Cermet resistive element for variable resistor

Abstract

An improved resistive element comprises a film-type resistive layer applied to an insulative substrate and then fired. An array of discrete, spaced apart islands of predominantly conductive material is then applied to the resistive layer in a repetitive pattern having predetermined inter-island spacing. The islands have a conductivity that is substantially greater than the conductivity of the resistive layer. Preferably, the islands are of substantially uniform shape and size. In one preferred embodiment, the islands are formed of a conductive thick film ink that is screen-printed onto a cermet resistive layer through an appropriate mask, and then fired. In another preferred embodiment, the islands are formed of a conductive metal that is applied to the resistive layer by vapor deposition, sputtering, or ion implantation through a suitable mask. Either embodiment of the invention provides a resistive element with lower contact resistance and improved contact resistance stability than prior art film-type resistive elements, while maintaining good linearity, setability, and resolution.

What is claimed is: 
     
       1. A resistive element for a variable resistor or the like, comprising: an insulative substrate;   a layer of resistive film on said substrate, said resistive film being substantially of a material selected from the group consisiting of cermets and carbon-filled polymers; and   an array of discrete, spaced-apart islands of predominantly conductive material formed on the surface of said layer in a repetitive pattern of predetermined spacing, said predominantly conductive material being selected from the group consisting of high metal content thick film inks, high metal oxide content thick film inks, substantially pure conductive metal, and carbon-filled polymers, whereby the interfaces between said islands and said layer form electrically conductive junctions between said islands and said layer.   
     
     
       2. The resistive element of claim 1, wherein said resistive film is cermet. 
     
     
       3. The resistive element of claim 1, wherein said islands are substantially of uniform size and shape. 
     
     
       4. The resistive element of claim 2, wherein said islands are formed of a high metal content thick film ink. 
     
     
       5. The resistive element of claim 4, wherein said thick film ink has a metallic component selected from the group consisting of gold, silver, and silver/palladium alloy. 
     
     
       6. The resistive element of claim 5, wherein said islands are predominantly a material selected from the group consisting of ruthenium dioxide and lead ruthenate. 
     
     
       7. The resistive element of claim 1, wherein said islands are substantially pure conductive metal. 
     
     
       8. The resistive element of claim 7, wherein said metal is a noble metal. 
     
     
       9. The resistive element of claim 7, wherein said metal is an alloy of nickel and chromium. 
     
     
       10. A resistive element for a variable resistor or the like, comprising: an insulative substrate;   a layer of resistive film on said substrate, said layer having a first conductivity; and   an array of discrete, spaced-apart islands on the surface of said layer in a repetitive pattern with predetermined spacing, said islands being formed of a material having a second conductivity which is greater than said first conductivity.   
     
     
       11. The resistive element of claim 10, wherein said repetitive pattern has substantially uniform spacing. 
     
     
       12. The resistive element of claim 10, wherein said resistive film is a cermet. 
     
     
       13. The resistive element of claim 12, wherein said islands are formed from a high metal content thick film ink. 
     
     
       14. The resistive element of claim 13, wherein said thick film ink has a metallic component selected from the group consisting of gold, silver, and silver/palladium alloy. 
     
     
       15. The resistive element of claim 14, wherein said islands are predominantly a material selected from the group consisting of ruthenium dioxide and lead ruthenate. 
     
     
       16. The resistive element of claim 11, wherein said islands are of substantially uniform shape and size. 
     
     
       17. The resistive element of claim 10, wherein said islands are formed substantially from a noble metal. 
     
     
       18. The resistive element of claim 10, wherein said islands are formed substantially from an alloy of nickel and chromium. 
     
     
       19. A resistive element for a variable resistor or the like, comprising: an insulative substrate;   a cermet layer on said substrate; and an   array of discrete islands of predominantly conductive material formed on the surface of said cermet layer in a repetitive pattern, said islands being of substantially uniform size and shape, with a conductivity greater than the conductivity of said cermet layer.   
     
     
       20. The resistive element of claim 19, wherein said islands are formed of a high metal content thick film ink. 
     
     
       21. The resistive element of claim 20, wherein said thick film ink has a metallic component selected from the group consisting of gold, silver, and silver/palladium alloy. 
     
     
       22. The resistive element of claim 21, wherein said islands are predominantly a material selected from the group consisting of lead ruthenate and ruthenium dioxide. 
     
     
       23. The resistive element of claim 19, wherein said islands are substantially pure conductive metal. 
     
     
       24. The resistive element of claim 23, wherein said metal is a noble metal. 
     
     
       25. The resistive element of claim 23, wherein said metal is an alloy of nickel and chromium. 
     
     
       26. The resistive element of claim 19, wherein said cermet layer is a thick film. 
     
     
       27. The resistive element of claim 2, wherein said islands are formed of a high metal oxide content thick film ink. 
     
     
       28. The resistive element of claim 1, wherein said resistive film is a first carbon-filled polymer having a first conductivity, and wherein said islands are formed of a second carbon-filled polymer having a second conductivity higher than said first conductivity. 
     
     
       29. The resistive element of claim 10, wherein said resistive film is a carbon-filled polymer. 
     
     
       30. The resistive element of claim 29, wherein said resistive film is formed from a first carbon-filled polymer having a first conductivity, and wherein said islands are formed of a second carbon-filled polymer having a second conductivity higher than said first conductivity. 
     
     
       31. The resistive element of claim 12, wherein said islands are formed from a high metal oxide content thick film ink. 
     
     
       32. The resistive element of claim 19, wherein said islands are formed from a high metal oxide content thick film ink.